1. Field of the Invention
The invention concerns an inorganic adhesive composition intended for bonding porous to non-porous metallic materials. More particularly, it relates to an adhesive composition with which it is possible to achieve bonded assemblies offering high resistance to shearing when heated up to temperatures of the order of 600.degree. C. and which are also capable of resisting repeated temperature variations (heat cycles).
The resistance to shearing at elevated temperatures which creates bonded assembly problems of this type are experienced particularly when it comes to forming a tight seal between the successive compression stages of a compressor making up part of a turboshaft engine in order to provide the seal required to improve the thermodynamic efficiency.
Turbojet manufacturers provide for the installation of sealing joints inside casings in the space separating them from the tips of the rotor blades as well as for labyrinth joints on the rotating shafts. The blade tip seals must be abradable in such a way as not to wear away the tips of the blades which are in contact with the seals, whereas the rotating shaft seals must have plastic qualities for fulfilling their role.
2. Description of the Prior Art
Several adhesive materials are suitable for making up the seals but the ones performing best, which offer the advantage of combining the abradable and plastic qualities required for both uses mentioned, are metallic felts of nickel or nickel-chromium composed of a criss-cross of hollow fibres of a few microns in diameter as described in the French Pat. No. 2,158,732 filed on Sept. 23, 1969 and the French Pat. of addition No. 2,285,475 filed Sept. 17, 1974. The extreme fineness of the pores of these felts and their high level of porosity make it possible to obtain exceptional resistance to the passage of gas while obviating damage to the opposed parts.
The peak temperatures reached by the felts secured in the casing are around 600.degree. C., the heat cycle being dependent on the conditions of use of the engines. The secure attachment of felts to the casing wall is, thus, a problem which is difficult to solve.
One solution consists in attaching the felts by brazing, but the alloys making up the casings always contain chromium to ensure corrosion protection. This high temperature of oxide formation of this element disturbs the absorption of the brazing to such an extent that exceptional precautions, such as brazing in high vacuum or by using a balanced hydrogen-helium atmosphere, have to be taken. When the casings are on the large size, the equipment is expensive and a considerable amount of energy is consumed.
Furthermore, if a defect is turned up during inspection, the casing seal assembly often has to be scrapped, since the successive brazing operations subject the casings to overheating likely to give rise to unacceptable geometric deformations. Another solution consists in bonding the felt skin by means of a layer of inorganic adhesive such as, for example, those known under the trade mark SERMETAL OHC 732 or 481.
Products of this inorganic adhesive type contain a mineral binder giving the necessary qualities of adhesion and a powdered metallic filler generally aluminium powder based whose role is to impart plasticity to the product, after applying a suitable thermal bonding treatment, the thermal treatment enhancing the considerable plasticity allowing it to bear the stresses caused by heat.
In this solution, fixing the components of the inorganic adhesive composition is achieved by a straight-forward heating process at 350.degree. C. which transforms the product into a metallo-ceramic cement which is highly adherent and strongly resistant to heat cycles up to 450.degree. C. This method of assembly affords many advantages, such as ease of preparation and use of the adhesive, simple and cheap equipment (clamps, oven) together with the absence of any scrap since the support component can be recovered by removing the adhesive seal upon dipping into a hot sodium solution. However, for temperatures greater than 450.degree. C., the metallo-ceramic cement rapidly loses its adherence properties following corrosion of the aluminium filler particles.
The adhesive thus cannot solve the problems experienced in practice for attaching sealing felt in the last stages of turboshaft compressors or in the corresponding labyrinths where the temperature of the components exceeds 450.degree. C. and can reach around 600.degree. C.
One is thus forced to resort to the afore-mentioned brazing methods and to bear the related drawbacks.